Langmuir Films of n-Alkanethiol-Capped Gold Nanoparticles and n

Jan 17, 2018 - As islands of nanoparticles are gathered and compressed, the interdomain correlation length is reduced as the nanoparticle domains are ...
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Article Cite This: J. Phys. Chem. C 2018, 122, 2975−2982

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Langmuir Films of n‑Alkanethiol-Capped Gold Nanoparticles and n‑Alkanes: Interfacial Mixing Scenarios Assessed by X‑ray Reflectivity and Grazing Incidence Diffraction A. Raveendran,† C. DeWolf,‡ W. Bu,§ S. McWhirter,† M. Meron,§ B. Lin,§ and M.-V. Meli*,† †

Department of Chemistry and Biochemistry, Mount Allison University, Sackville, New Brunswick E4L 1G8, Canada Department of Chemistry and Biochemistry and Centre for NanoScience Research, Concordia University, Montreal, Quebec H4B 1R6, Canada § ChemMatCARS, University of Chicago, Chicago, Illinois 60637, United States ‡

ABSTRACT: A series of n-alkanes cospread with alkanethiolstabilized gold nanoparticles (AuNP) were studied as Langmuir monolayers by synchrotron X-ray reflectivity and diffraction. Tetradecanethiol capped gold nanoparticles with core diameters close to 2 nm were used to make films at 20 °C, below the ligand order−disorder temperature. A variety of n-alkane chain lengths (Cn = CnH2n+2, where n = 12, 15 and 16) were tested to assess the interfacial assembly of nanoparticle films as a result of different mixing scenarios indicated in their surface pressure versus area isotherms. Synchrotron grazing incidence X-ray diffraction (GIXD) and reflectivity (XR) confirm that mixtures of n-alkane and AuNP exhibiting improved fluidity in their compression isotherm are indeed incorporating n-alkane into the AuNP ligand shell and stabilizing it at the air−water interface. The resulting films show a doubling of their correlation lengths and thus a significant improvement on their ordering, as well as increased lattice spacing that is dependent upon the n-alkane chain length. Mixtures that do not exhibit changes in their surface pressure vs area isotherm similarly show little change in the interfacial assembly of the nanoparticle films except to promote monolayer collapse and multilayer formation. Improvements to the film order are assigned to the initial formation of larger nanoparticle domains. The nature of the chain length dependence and persistence of the n-alkane through compression suggest a favorable interaction with the nanoparticle ligand shell that results in the extension of methylene units of the longer alkanes beyond the thiol layer, which has implications for influencing nanoparticle interactions.

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INTRODUCTION

Interfacial self-assembly is at the heart of some of the best examples of 2D superlattice formation.6,16,17 The study of such systems under compressive stress using the Langmuir balance enables a dynamic view of the interfacial assembly of nanoparticles.14,18−22 In a previous investigation into Langmuir films of alkanethiol-capped AuNPs mixed with n-alkane (Cn = CnH2n+2), the ligand phase state (where the ordered state was achieved at T < Tm, the literature phase transition temperature) as well as the n-alkane length (where nalkane > nalkylthiol) determined whether a liquid-like nanoparticle monolayer could be obtained.23 When these two conditions were not met, the presence of n-alkane at the interface had no effect on the isotherm behavior and resulted in solid-like compression as seen in pure AuNP films. Determining how ligand phase and alkane chain length indeed affect nanoparticle−solvent interactions in these systems can provide a new parameter for controlling nanoparticle assembly. The presence of a homogeneously mixed fluid phase, where nanoparticles are

The interfacial assembly of ligand-stabilized nanoparticles is of interest for enabling the bottom-up organization of nanoparticles into next-generation 2D and 3D materials.1−6 The interaction of the ligand shell with its surrounding environment is a key parameter in nanoparticle self-assembly, yet even welldefined nanoparticle−ligand systems can reveal surprising trends in their ligand-controlled properties, such as solubility,7 surface reactivity, and interfacial behavior. For example, mixed ligand shells consisting of both hydrophobic and hydrophilicterminations (−CH3 and −OH respectively) have been found to give rise to nonmonotonic changes in nanoparticle hydrophobicity;8−10 chain packing frustration has been shown to determine the coexistence of bcc and fcc close-packed 3D superlattices;11 and variations in ligand density on gold nanorods have been shown to control regiospecific chemistry at the nanoparticle surface.12 Recently, the importance of ligand phase in determining nanoparticle−interface and nanoparticle− nanoparticle interactions has come to light, leading to improved ordering in 3D superlattices13 and control over the compressive and tensile strength of 2D superlattices.14,15 © 2018 American Chemical Society

Received: October 5, 2017 Revised: January 13, 2018 Published: January 17, 2018 2975

DOI: 10.1021/acs.jpcc.7b09874 J. Phys. Chem. C 2018, 122, 2975−2982

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The Journal of Physical Chemistry C individually separated by regions of n-alkane, could enable reversible self-assembly of these films. However, while the emergence of a liquid-like compression regime implies additional surface-active material at the interface, the mixing within this fluid phase monolayer and the role of the n-alkane during monolayer compression are unclear. High resolution in situ liquid surface X-ray scattering studies of Langmuir films enable a detailed characterization of the nanoparticle spacing, ordering, and vertical positioning, which can provide insight into the role of the ligand during assembly and during compression. In an early study of gold nanoparticles with mixed length alkylthiol ligand shells, Nørgaard et al. suggested a dynamic ligand redistribution as a function of compression.24 Several studies have investigated the response of nanoparticle films to compressive stress at the air−water interface,25−27 including the nature and reversibility of the film transition from monolayer-to-bilayer (in the case of 2 nm acidderivatized alkylthiol AuNPs) and monolayer-to-trilayer (in the case of 6 nm dodecanethiol-capped AuNP).26,27 More recent investigations have involved nanoparticles mixed with other components in order to affect interfacial assembly28 or assess nanoparticle activity on film properties.29 In this study, we use X-ray reflectivity (XR) and grazing incidence X-ray diffraction (GIXD) of Langmuir films of ∼2 nm tetradecanethiol-capped gold nanoparticles mixed with n-alkanes (n = 12, 15 and 16), to elucidate the nature of these n-alkane/nanoparticle mixed Langmuir films, the role of the n-alkane, and its implications on the film assembly process.

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EXPERIMENTAL SECTION The synthetic procedures in preparing tetradecanethiol-capped gold nanoparticles have been described previously.23 Chloroform (HPLC-grade) and n-alkanes (≥99%) were purchased from Sigma-Aldrich Canada and used as received, and ultrapure water (18 MΩ resistivity, Millipore) was used as a subphase. Surface pressure vs area isotherms were obtained with a Teflon Langmuir trough of dimensions 8.9 cm × 42 cm equipped with a filter paper Wilhelmy balance.26 The trough is enclosed in a hermetically sealed box and the interior is purged with helium to reduce the absorption of X-rays by air. In the case of the tetradecanethiol-capped gold nanoparticles (concentration 0.78 mg AuNP per 1 mL chloroform), 400 μL of the solution was spread at the air−water interface dropwise (3−5 μL drops every 15 s), and for the Cn-mixed films (concentration 0.70 mg AuNP per 1 mL of 5% Cn (v/v) solution in chloroform), 450 μL was spread. Deposition and compression of the films were separated by 30 min to allow for complete solvent evaporation. The compression was performed at a rate of 1.7 nm2min−1NP−1 and experiments were performed at a temperature of 20 °C. The X-ray reflectivity (XR) and grazing incidence X-ray diffraction (GIXD) measurements were conducted at the Advanced Photon Source (Argonne, IL) in ChemMatCARS, station 15-ID-C, using monochromatic X-rays with a wavelength of 1.24 Å (10 keV). The experimental geometry has been illustrated and described in detail previously.30,31 Both XR and GIXD measurements were performed on all the films at various surface pressures along the isotherm. Films were compressed to a desired surface pressure, and then held at constant area for the duration of the XR and GIXD measurements. Holding the barriers constant led to decreases in the surface pressure during this time due to film relaxation as indicated in Figure 1B; however, the film surface pressure would stabilize and would increase along the same slope once

Figure 1. TEM image of tetradecanethiol-capped gold AuNP (scale bar: 50 nm) and corresponding size histogram. Representative compression isotherm of solid-like AuNP film (gray) and liquid-like film of AuNP + C16 (black). Points represent typical reductions in surface pressure observed at various points in the compression due to film relaxation.

compression was resumed, as has been seen in other studies.27,32 Results presented are for measurements taken once the surface pressure stabilized, changing by less than 2 mN/m over the course of the hour-long XR measurements. However, close to collapse the films are inherently unstable, and in these cases the surface pressure dropped more significantly (>5 mN/m) over the course of the XR measurement. It was noted that GIXD spectra taken before and after a relaxation event were essentially the same, indicating that the relaxation occurs mainly between interdomains rather than intradomain. XR measurements, which probe the monolayer structure in the direction normal to the air−water interface (z-axis) were analyzed to generate an electron density profile along this direction using the open source software provided by the beamline. Details for the slab model and fitting procedure used in the program have been reported previously. 33 Each layer is represented by a box of independently varying thickness l, roughness, and relative electron density ϕ = ρlayer/ρwater, where ρwater = 0.333 e/Å3. The best fit model was determined by the least-squares fitting yielding the lowest χ2 value. From the box fits, the typical size of the gold core was estimated from the thickness of the gold layer (l). Considering the gold cores as uniform spheres, and 2976

DOI: 10.1021/acs.jpcc.7b09874 J. Phys. Chem. C 2018, 122, 2975−2982

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The Journal of Physical Chemistry C from the FWHM, Δz = D/√2, which is the projection of the sphere on the z axis, the gold core diameter (D), can be estimated from D = √2Δz = √2l. The GIXD measurements provide information about nanoparticle symmetry and spacing within the plane of the air− water interface. The Bragg peaks were analyzed to determine 2π the lattice spacing d given by d = q and the nearest neighbor xy

2d 3

. The correlation length, ξ, of the distance a, given by a = scattering domains was determined using the Scherrer 0.9 formula,19 ξ = 2π × FWHM . In our system of ca. 2 nm gold qxy

nanoparticle (AuNP) monolayers, d is expected to be in the range of 20 Å to 60 Å. According to Bragg’s law, the scattering angle, θ, is expected to be in the range of 0.6° to 1.8° for the first order diffraction peak, hence, in our experiment, θ was scanned from 0.5° to 3.5° in 60 steps, with a data acquisition 4π time of 10 s at each step. According to qxy = λ sin θ , qxy is

expected in the range of 0.09 Å−1 to 0.25 Å−1.

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RESULTS The core size of the tetradecanethiol-capped gold nanoparticles was estimated from transmission electron microscopy (TEM) images to yield an average diameter (±standard deviation) of 2.1 ± 0.5 nm (Figure 1A). As described previously, we estimate the number of nanoparticles spread at the air−water interface14 by assuming 1:3 thiol:surface Au atoms on a sphere of equivalent diameter. Langmuir isotherms were confirmed to be consistent with those of the previous study23 for both types of mixtures: a solid-like film resulting from poor mixing, which follows that of the pure nanoparticle film, and a liquid-like film resulting from better mixing with n-alkane(Figure 1B). Representative reflectivity scans, normalized to the Fresnel reflectivity for an ideal interface (Rf), of the films at low surface pressure are shown in Figure 2a. In the alkane-free tetradecanethiol-capped gold nanoparticle (AuNP) film, we observe a characteristic curve similar to previous studies: two large amplitude maxima and a minimum, which persists throughout the film compression (data not shown) until high surface pressures, indicating that the film thickness is fairly constant through the compression. The amplitude of the first maximum increases slightly with initial compression, indicating more complete surface coverage, as nanoparticle islands that self-assemble during the spreading and evaporation process are brought together to form a contiguous layer. In comparison, the AuNP+Cn mixtures generate more curvature in the reflectivity curve, indicating increasing film thickness. The amplitude of the first maximum (at qz ∼ 0.10−0.15 Å−1) decreases dramatically with the longer n-alkane incorporated, indicating that the density of the film is decreasing compared to the (alkane-free) AuNP film. Upon initial compression, the amplitude of the first maximum of the n-alkane mixtures also increases slightly, indicating more complete surface coverage by the AuNP, however, it remains less than that of the (alkane-free) AuNP monolayer throughout the compression, indicating the persistence of alkane in the nanoparticle monolayer. In generating the best-fit curves, the AuNP data were adequately fit using a single box model as used in other studies of nanoparticle interfacial films.27,32,34,35 However, for comparison across all films, a three-layer system (organic−Au− organic) was employed, where the organic layers were expected to contain tetradecanethiol ligand (exclusively, in the case of

Figure 2. X-ray reflectivity of the AuNP film compared to AuNP−Cn mixtures at low compression (0−2 mN/m) (A) and corresponding electron density profiles in (B) denoted as follows: □ AuNP, ● C12, △C15, and ▲C16. AuNP−C16 at various surface pressures (C). Plots are offset vertically in (A) and (C) for clarity.

AuNP film) or ligand + n-alkane oil (Table 1). Such an approach allows the determination of n-alkane distribution throughout the interfacial region, rather than assuming the interface is static and estimating nanoparticle contact angle, as has been useful in other studies.34,35 The roughness of each layer was allowed to vary freely, and generally the layers had values in the range of 0.2−0.4 nm for all films except the C12 mixtures, which ranged from 0.2−0.7 nm. These values are reasonable considering roughness arising from thermally excited capillary waves is expected to be 0.25−0.31 nm.36 Typical electron density plots are shown in Figure 2b, where 2977

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The Journal of Physical Chemistry C Table 1. XR Parameters Obtained from Model Fits for Films Prior to Monolayer Collapsea film

π (mN/m)

AuNP

2 9 2 9 2 9 2 9 11

C12 C15 C16

ϕ1 1.03 1.01 1.3 1.4 1.0 1.1 0.9 1.2 1.0

± ± ± ± ± ± ± ± ±

ϕ2

l1 (nm) 0.04 0.01 0.2 0.1 0.3 0.1 0.3 0.1 0.2

0.93 1.10 1.1 1.0 1.1 1.9 0.9 4.2 2.9

± ± ± ± ± ± ± ± ±

0.10 0.00 0.1 0.1 0.1 0.1 1.2 3.6 2.5

4.92 5.26 4.6 6.7 3.3 3.3 2.9 3.5 4.9

± ± ± ± ± ± ± ± ±

l2(nm) 0.03 0.07 0.2 2.3 0.6 1.1 0.3 0.2 1.5

1.45 1.44 1.45 1.37 1.42 1.40 1.45 1.4 1.35

± ± ± ± ± ± ± ± ±

0.00 0.02 0.03 0.06 0.03 0.02 0.01 0.2 0.05

l3 1.30 1.22 1.5 1.12 0.80 1.1 0.9 0.91 0.8

± ± ± ± ± ± ± ± ±

l3 (nm) 0.07 0.02 0.4 0.04 0.02 0.2 0.3 0.04 0.3

0.96 1.17 1.3 1.6 1.0 1.02 1.91 1.87 2.2

± ± ± ± ± ± ± ± ±

0.01 0.04 0.3 0.5 0.2 0.02 0.08 0.03 0.1

DAu (nm) 2.05 2.03 2.05 1.94 2.01 2.0 2.04 1.9 1.90

± ± ± ± ± ± ± ± ±

0.00 0.02 0.04 0.08 0.04 0.3 0.01 0.2 0.07

Average values ± standard deviations of two replicate film measurements obtained to within 1 mN/m of the surface pressures (π) listed. Normalized electron density (ϕ), layer thickness (l) of the layers 1−3 correspond to the water-adjacent organic layer, gold layer, and air-adjacent organic layers respectively. DAu is the estimated diameter of the nanoparticles based on the layer 2 thickness. a

become more variable. Taken together with the reduced quality of the fit for these films, greater roughness values obtained, and the TEM evidence seen in our previous study,23 this data suggests that these films consist of both monolayer and some multilayer regions. This interpretation would account for the larger than expected electron density values seen in layer 3. By contrast, upon incorporation of longer n-alkanes (C15 and C16), the electron density of the Au layer is significantly reduced, especially at lower surface pressures, while the thickness of this layer remains constant (see Figure 3). This

the majority of the electron density is located within the middle layer as a peak, the width of which corresponds well with the nanoparticle diameter in each case. In modeling the AuNP film, the relative electron density values for the Au layer increased from 4.89 to 5.19 upon compression from 2 to 8 mN/m, while that of the thiol was estimated to be ∼1. These values compare well with the literature, where Fukuto et al. obtained a value of 5.6 and Lin et al. obtained a value ∼6.27,32 The ϕ2 values also suggest that the defect-rich film morphology seen previously32 is present in these pure AuNP films as well. As nanoparticles close-pack, multiple nearest-neighbor interactions result in the formation of a network of nanoparticle domains, rich with compression-resistant void defects, until monolayer collapse eventually occurs. Using the estimate of the Au layer thickness for the AuNP films, the average Au core diameter D was determined to be 2.04 nm, in agreement with the TEMdetermined core size. This treatment yielded normalized electron densities of the air-adjacent layer greater than 1.0, an artifact likely arising from the use of a box model to represent a spherical nanoparticle electron density profile. Upon cospreading 5% n-alkanes (Cn) with the gold nanoparticle films, the nature of the Cn-nanoparticle interaction appears to play a significant role on the organization of the nanoparticle films (Figure 2a), as suggested in our earlier study.23 In particular the C16 mixtures are quite different from the rest of the systems studied, with an additional maximum at qz ∼ 0.29 Å−1. All of the films were measured throughout their compression isotherm (see Figure 2c for example) until evidence of collapse was noted in the reflectivity data. For example, in Figure 2c, the mixture of C16 with the AuNP film shows very modest changes in the reflectivity spectrum until a surface pressure of 20 mN/m, whereupon a striking increase in the number of reflectivity peaks occurs that is reminiscent of curves associated with collapse seen in other studies.27,32 Also shown in Figure 2 are the best fits obtained using the threelayer model which are generally good for all but the highest qz values. As seen in Figure 2a, the C12 mixture has a relatively poor fit using the three-layer model; comparison with the collapse curve seen in Figure 2c for the C16 mixture suggests that the C12 mixed film has emerging collapsed (multilayer) regions. Table 1 lists the obtained fit values, where layer 1 is adjacent to the water subphase, layer 2 corresponds with the Aucontaining layer, and layer 3 is adjacent to the surrounding air. The tabulated results are limited to those obtained before monolayer collapse. When C12 is included in the films, the electron densities and thicknesses of all three layers tend to

Figure 3. Schematic representations of chain length-dependent changes in Langmuir films of tetradecanethiol-capped AuNP+C12 (left) vs AuNP+C15 or C16 (right).

result suggests that while the nanoparticle core diameter remains constant upon incorporation of C15 and C16 and compression, the nanoparticle films must form with n-alkane between the particles to account for the surface pressures at lower areal density. In the C15 and C16 mixtures, the electron densities of the water-adjacent organic layer (layer 1) become more variable yet remain in agreement with that of an alkane, and the thickness of this layer becomes quite variable in the C16 mixture. In other words, the lateral distribution or density of alkane (from both thiol and n-alkane) present in the first interfacial layer does not change significantly, suggesting that as n-alkane is incorporated into the mixed film, it compensates for the lower electron density expected in layer 1 as the nanoparticles become less densely packed. Through interaction with the alkylthiol ligand caps, the n-alkane is in effect stabilized at the air−water interface, rendering it surface active, giving rise to larger compression onset areas (Figure 1B), as was shown for lipid/n-alkane mixtures at the air−water interface.37 Interestingly the layer 1 thickness at low compression is more variable for C16 than C15, suggesting that the presence of C16 in the water-adjacent layer is more irregular. This may follow earlier results suggesting C16 to be less surface active than C15 2978

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The Journal of Physical Chemistry C when spread at the air−water interface at this temperature,23 and thus depends more strongly upon the presence of the gold nanoparticle domains. In contrast to layer 1, the air-adjacent layer (layer 3) thickness values have relatively low variability, which indicates that a smooth and continuous layer is formed. The electron density values for this layer in C15 and C16 are generally in agreement with that of pure alkane, suggesting that electron density originating from the alkanethiol is replaced with n-alkane as it integrates into the film. Furthermore, layer 3 doubles in thickness for C16 and remains unchanged for C15, and both remain nearly constant upon compression. This is a significant difference from the trend seen in layer 1, as layer 3 should be less dependent upon the presence of water, and thus the incorporation of n-alkane into this layer depends more simply upon packing constraints and the favorability of the interaction between the nanoparticles and n-alkanes. These results suggest that the interaction of the nanoparticles with C16 is much more favorable than with C15, which will be discussed further in the context of the GIXD results. Representative GIXD spectra for the different films at low surface pressure (0−2 mN/m) are shown in Figure 4 with the

Figure 5. (A) GIXD of the AuNP film compared to AuNP+Cn mixtures at low compression (0−2 mN/m). (B) AuNP+C16 at various surface pressures. Plots are offset vertically for clarity.

is no indication of a change in the packing (lattice spacing) of the nanoparticle film. However, Figure 5a indicates that the C15 and C16 alkanes have a more profound effect on the nanoparticle film organization: the peak shape is sharpened dramatically, indicating an improvement in the ordering to such an extent as to give rise to the second order peak (at qxy = 0.30 Å−1, and 0.29 Å−1, respectively); furthermore the first order peak of the C15- and C16-laden film are shifted to lower values of qxy, which indicates an increase in the lattice spacing (Figure 5a). Figure 6 compares the mean lattice a-spacing as well as correlation coefficient obtained from the GIXD spectra, at various surface pressures (note that all films have indicated collapse in the XR spectra by the 20 mN/m measurement). The incorporation of the longer alkanes leads to significant increases in lattice spacing at low surface pressures (Figure 6A, blue) corresponding to incorporation of alkane within the nanoparticle lattice plane, effectively expanding the edge-toedge spacing (calculated by a − D) by 30% on average. This appears to be the primary source of the large increase in the onset area seen in the isotherm at 1−2 mN/m upon addition of C16 (Figure 1B). As the films are compressed to 9 mN/m (Figure 6A, orange), no significant dependence is seen in the nearest neighbor spacing for most of the films, in agreement with previous studies of thiolated AuNP Langmuir films.27,32

Figure 4. Raw GIXD detector signals obtained at low compression (0−2 mN/m) for AuNP film (A), AuNP + C12 (B), AuNP + C15 (C), and AuNP + C16 (D). Arrows in panels C and D indicate the first- and second-order peaks.

best Lorentzian fits for these and for additional points along the compression isotherm provided in Figure 5. The observed GIXD pattern for the AuNP film is as expected for 2.0 nm nanoparticles capped with a ligand layer (approximately 1 nm thick) and compares well with the literature.24,25,27 The size polydispersity of these nanoparticles (∼20%) and typically defect-rich hexagonal packing limits the pattern to a single peak at qxy = 0.18 Å−1. Similar to previous studies, the AuNP films display a peak whose position is nearly independent of the surface pressure (data not shown), even upon monolayer collapse at high surface pressures.27,32 Upon addition of nalkane, however, significant changes are observed depending upon the chain length (Figure 4). There is a slight sharpening of the peak shape when C12 is spread with the nanoparticles, indicating an improvement in the organization; however, there 2979

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The Journal of Physical Chemistry C

estimated to add 1.26Å/CH2 to each neighboring nanoparticle to yield an estimated change in a-spacing of ∼2.5 Å/CH2, suggesting that the n-alkane packs efficiently into the ligand shell such that the extraneous methylene units extend beyond the alkylthiol shell. These results suggest that both alkanes are incorporated into the close-packed nanoparticle lattice to a limited extent (up to 1 layer of alkane per nanoparticle) and that changes in the lattice spacing originate from the difference in length between the n-alkane and the nanoparticle ligand layer. Unlike C12 or C15, the C16, with two methylene units extending from the ligand shell, has greater degrees of freedom to rotate the extended units in response to compression, thus leading to the observed decrease in nearest neighbor distance. Implications on Nanoparticle Assembly. In comparison to work by Schultz et al. investigating the addition of excess thiol to a Langmuir film of 6 nm gold nanoparticles,32 the presence C15 and C16 has a more modest effect on the edgeto-edge spacing, but a much larger effect on the correlation length. Unlike evaporative assembly from a droplet on a solid support, assembly at the air−water interface involves the rapid evaporation of solvent spread into a thin film over a large surface area, typically leading to the nucleation of many small hydrophobic domains, which group together irreversibly to form void-containing island domains. Our correlation lengths suggest that the small AuNP domains typically consist of at least 2−3 nanoparticles, and that this doubles upon addition of C16. These small domain sizes are expected for these smaller core sizes used,27,38 where attractive van der Waals forces between the cores are much less significant than in the 6 nm cores, thus slowing the initial assembly of domains that occurs during solvent evaporation. It is interesting to note that the addition of excess thiol studied by Schultz et al. also led to an increase in domain size by 1−2 nanoparticles, suggested to be a result of an increased ligand density and steric repulsion, and thus more reversible assembly. Similarly, the mixing of C15 and C16 clearly increases the steric repulsion given the larger aspacing measured, and may also promote the formation of larger domains by lowering the interfacial tension and spreading coefficient. Once formed, the domains nonetheless are gathered to form glassy islands, as suggested by the attenuated correlation lengths upon compression (Figure 6b). Incorporation of n-alkane within the ligand shell would both prevent its removal via compression and improve the reversibility of island assembly particularly at lower surface pressures, as was evidenced earlier in compression−expansion hysteresis measurements.23 Finally, recalling that increases in the air-adjacent layer (layer 3) thicknesses were observed for C16 but not C15 (Table 1), we speculate that this may indicate a stronger interaction between C16 and the tetradecanethiol ligand shell, akin to odd−even effects observed in wettability studies of n-alkanethiol monolayers on gold.39 The strong XR signal obtained from the gold nanoparticle core prevents detection of such changes in the organic ligand shell.40 The observed changes in the lattice spacing seen in this study thus originate mainly from the difference in chain length between the n-alkane and the nanoparticle ligand layer, but future investigations will be made into the subtle differences in the interactions between C16 with the alkanethiol nanoparticle shell compared to C15.

Figure 6. (A) Nearest neighbor spacing, a (nm), and (B) correlation length (nm), of two replicate measurements of AuNP and AuNP+Cn films at various stages of compression: 1 mN/m (blue), 9 mN/m (orange), 20 mN/m (yellow). Note that at 20 mN/m all films have shown XR evidence of monolayer collapse. Measurements were obtained to within 1 mN/m of the surface pressure listed.

However, unlike a previous study which did not find any compression-dependence when excess thiol is mixed with the AuNP monolayer,32 the nearest neighbor distance decreases slightly with compression for C16 mixed films, which we interpret below. Notably, the correlation length at low surface pressure (Figure 6B, blue) is nearly doubled upon incorporation of both the C15 and C16, indicating improved ordering within the domains. In agreement with previous studies of nanoparticle Langmuir films, the ordering is diminished with increasing surface pressure,27,32 as indicated by a broadening of the peak and loss of the second order peak (Figure 5B). As islands of nanoparticles are gathered and compressed, the interdomain correlation length is reduced as the nanoparticle domains are unable to rearrange significantly to correct defects formed, but are strained and start to collapse at higher compression. The increased spacing in the C15 and C16-laden films persist throughout the compression, despite being diminished in the case of C16, suggesting that the incorporated alkane is not removed from between the nanoparticles. The lower electron density values seen for layer 2 of the C15 and C16-mixed films (Table 1), but not for C12-mixed films, suggest that the longer alkanes give rise to a smaller ratio of Au:organic in the nanoparticle plane. In other words, at the same surface pressure, the longer alkanes effectively lower the nanoparticle areal density, while the C12 does not. This chain-length dependence implies that the n-alkane does not merely surround the ligand shell, but rather, is incorporated into it. The nearest neighbor a-spacing observed for C15 and C16 amounts to 2−3 Å per additional methylene unit in the n-alkane relative to the tetradecanethiol ligand cap. A fully extended alkyl chain can be

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CONCLUSIONS We have shown that below the ligand order−disorder temperature, mixtures of alkanethiol-capped gold nanoparticles 2980

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The Journal of Physical Chemistry C and n-alkanes that exhibit fluid phase Langmuir isotherms incorporate the n-alkane into the nanoparticle lattice, increasing interparticle spacing and improving order. The correlation lengths of the resulting lattice are doubled, reflecting an improvement on the assembly of the initial domains, which subsequently assemble into glassy films upon compression. The minimum chain length required to observe this effect is determined by the ligand length. When below the minimum chain length, addition of alkane leads to destabilization of the Langmuir film to form multilayers and aggregates. The fluid phase observed in the corresponding Langmuir isotherms is a result of n-alkanes longer than the n-alkanethiol ligand incorporating into the ligand shell rather than around it, preventing its removal from the interface upon compression. The observed lattice expansion and improved ordering results from excess methylene units extending beyond the alkanethiol, improving the reversibility of the self-assembly by increasing the steric repulsion between particles. XR modeling of the airadjacent side of the interface suggests there may be a greater interaction between C16 and the ligand shell than C15. Future investigation will be aimed at clarifying the nature of this interaction as these insights could prove fundamental to predicting nanoparticle interactions with different media.

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AUTHOR INFORMATION

Corresponding Author

*Department of Chemistry and Biochemistry, Mount Allison University, Sackville, NB, Canada E4L 1G8. Phone: 506-3642363. FAX: 506-364-2313. E-mail: [email protected]. ORCID

B. Lin: 0000-0001-5932-4905 M.-V. Meli: 0000-0002-7715-9824 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS V.M. acknowledges support from Natural Sciences and Engineering Research Council (Grant Number 341933) and the Marjorie Young Bell Faculty Fund at Mount Allison University to perform this research. C.D. acknowledges support from Natural Sciences and Engineering Research Council (Grant Number 03977). ChemMatCARS Sector 15 is supported by the National Science Foundation under Grant Number NSF/CHE-1346572. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

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